Maskasky U.S. Pat. Nos. 5,264,337 and 5,292,632, hereinafter collectively referred to as Maskasky, disclose high chloride {100} tabular grain emulsions. The {100} tabular grains of Maskasky were "internally free of iodide", which is employed to mean no iodide ion is intentionally incorporated in the grains during their nucleation and growth prior to achieving their required tabular grain characteristics--i.e., prior to achieving an aspect ratio of at least 2. Maskasky in most instances relied upon as a grain growth stabilizer an organic compound containing a nitrogen atom with a resonance stabilized .pi. electron pair, although Example 2 ('337) Example 6 ('632) demonstrate that such a grain growth stabilizer is not essential.
Subsequently House et al U.S. Pat. No. 5,320,938 was able to demonstrate thinner and higher aspect ratio high chloride {100} tabular grain emulsions than demonstrated by Maskasky by performing grain nucleation in the presence of iodide ion. No restraining agents of the type disclosed by Maskasky were employed.
Chang et al U.S. Pat. No. 5,413,904 improved on the process of House et al by delaying iodide ion addition until after grain nuclei formation. By delaying iodide addition until after at least 0.01 percent of the total silver had been precipitated, Chang et al was able to significantly reduce the proportion of grains other than high chloride {100} tabular grains present in the emulsion. Additionally, the modified process was more robust, meaning that the process required minimum adjustments when the scale and equipment of precipitation was varied.